One closed-loop control principle which is often used, particularly in communications technology, such as for frequency demodulation and frequency multiplication, is phase locking (with a phase locked loop or PLL). In that process, the frequency of an oscillator is set in such a way that it agrees with a reference frequency, in fact so precisely that the phase displacement does not drift. Typical circuit configurations for phase locking have a controllable oscillator which is provided for this purpose and which issues an output signal that is compared with a reference frequency in a phase detector having an output signal which in turn sets the frequency of the controllable oscillator through a controlled system.
For instance, in a configuration known from an article by Floyd M. Gardner, entitled "Charge-Pump Phase-Lock Loops", in IEEE Transactions on Communications, Vol. COM-28, No. 11, Nov. 1980, a charge pump with a loop filter connected to the output side thereof is used as the controlled system. Upon the occurrence of a phase displacement between a reference signal and the output signal of the controllable oscillator, the charge pump is triggered by the phase detector in such a way that the loop filter is charged or discharged in pulsed fashion. In the locked state of the circuit configuration, in other words when virtually no phase displacement occurs, no further charge or discharge pulses are generated. The output signal of the loop filter, which is the voltage across the capacitor of a loop filter constructed as a passive RC element in the known circuit configuration, thus continues to be impressed, and no closed-loop control takes place. The jittering range of the entire configuration depends substantially on the internal gate transit times in the phase detector.
However, the resultant jittering is excessive for many applications.